Electrophoresis is a powerful technique for the separation of charged species in solution. Capillary zone electrophoresis (CZE) was first introduced by F. E. P. Mikkers and coworkers and later explored by J. W. Jorgenson and coworkers. Jorgenson developed electrophoresis processes with on-column detection. Research continues for new apparatus and methods for analyte characterization using electrophoresis and on-line detection. Heretofore most detectors have been optical detectors based on UV absorbance and fluorescence emission. Mass spectrometry is particularly suited for detection of CZE eluents with high sensitivity and selectivity.
Ion evaporation offers a preferred ionization mechanism for on-line CZE/MS because separated components already exist as charged species in the CZE buffer. Ion evaporation involves the emission of ions from the condensed phase into the gas phase and is a mild form of ionization for polar and ionic compounds. This process is one of the main mechanisms of ionization occurring in thermospray, fast atom bombardment, and electrospray ionization. Thermospray does not appear feasible for CZE/MS coupling due to the high liquid flowrates presently required for thermospray ionization and the apparent sensitivity limitations in comparison to other mass spectrometric techniques. Preliminary results demonstrating feasibility of coupling CZE and MS using a continuous atom bombardment (FAB) interface have been presented by R. D. Minard and coworkers (Paper ROC 945, 36th Annual Conference on Mass Spectrometry and Allied Topics June 5-10, 1988); however, excessive bandbroadening and loss of separation efficiency occurred due to the long transfer line to the FAB ion source and the associated high vacuum requirements. Wallingford and Ewing (Anal. Chem., 59:1762-1766, 1988) previously reported an interface for electrochemical detection based on a porous glass joint created in the column near the cathodic end of the CZE. The porous glass joint along with the cathode end of the capillary were submersed in a buffer reservoir for the purpose of permiting "the application of a potential gradient over one segment of capillary".
The electrospray ionization liquid interface for mass spectrometry developed by Fenn and coworkers has been used by R. D. Smith and coworkers to demonstrate the feasibility of CZE coupled to a mass spectrometer (Anal. Chem., 60:436-441 (1988)). In the Smith system the last few cm of the cathode end of the CZE capillary was sheathed with a stainless steel capillary and silver was vapor-deposited at the end of the two capillaries to make good electrical contact with the buffer flowing from the inside of the CZE capillary. The cathode end was used as the electrospray electrode for electrospray ionization. Although feasibility was demonstrated, this interface required high CZE electroosmotic flows and low buffer concentrations to maintain stable electrospray conditions.
Although the prior art systems have suggested the combination of CZE for separating multiple component analytes with mass spectrometer detection, such systems using an ion spray interface operated at atmospheric pressure suffer from band broadening of the separated species due to the discrepancy of flow rates between the zone electrophoresis separator (0.1 to 1.0 .mu.L/min flow) and the flow rates (1.0 to 50 .mu.L/min) required by the ion spray interface to accommodate the entire CZE eluent. The present invention relates to a fluid coupling of CZE and the ion spray LC/MS interface that allows for a wide range of electroosmotic flows without compromise of the CZE separation.